And for the unity of the Empire, methinks:
1. Emperor guarantees independence of all member states
2. All members states guarantee independence of Emperor
3. Emperor get's Money, IC and Manpower bonus
4. New Emperor are elected by event... Either historical (Just pick the emperor there is) or semihistorical (like two options and AI and human players vote through event
). Dont think it should be hard switching the bonuses and guarantees through such events
. Also, there's always the Grand Event Request 
1. Emperor guarantees independence of all member states
2. All members states guarantee independence of Emperor
3. Emperor get's Money, IC and Manpower bonus
4. New Emperor are elected by event... Either historical (Just pick the emperor there is) or semihistorical (like two options and AI and human players vote through event